We propose to study the synaptic organization of the neuronal circuits responsible for the generation of hippocampal theta rhythm. This is an ideal model system for the study of generation of rhythmic slow waves in brain. Knowledge of its mechanisms may also suggest hypotheses concerning its function. We will make intracellular recordings from hippocampal neurons during theta rhythm to assess the contributions of EPSPs and IPSPs. We will do current source density studies to localize the laminae with sources and sinks of current related to phase and/or presence or absence of theta rhythm. These data will be compared with existing anatomical and electrophysiological data in an attempt to identify the active synapses on the various phases of theta rhythm. If time permits, lesions will be made in regions which appear to be sources of afferents contributing to theta rhythm. Our long range goal is a quantitative model and a description of the cellular interactions for the generation of theta rhythm.